With a view to improving the fuel efficiency of a four-stroke engine and to lowering the air pollutants produced by the four-stroke engine, various improvements on the air-fuel ratio of the four-stroke engine have been made in the past. However, according to a number of tests, the current speed at which the engine air intake takes place is also an important factor affecting the fuel efficiency of the engine. It is known that the acceleration of the current of the engine air intake can bring about a strong turbulence in the engine cylinder, thereby resulting in an increase in the tumble ratio or the swirl ratio. As a result, the combustion variability of the engine is so reduced as to enhance the combustion efficiency of the engine.
The conventional method of improving the energy efficiency of a four-stroke engine includes an air duct which is located at the vicinity of the air intake valve and along the direction in which the valve stem is located. Such a design as described above can maximize the opening of the fuel gate, the quantity of air intake and the acceleration of the air current at such time when the work is done by the engine at a high load, thereby resulting in an output of greatest horsepower, as illustrated in FIG. 1A. However, when the engine is operating at an intermediate or low load, the amount of air intake and the speed at which the air current flows through the intake valve are reduced to an extent that the air current becomes stagnant in the engine cylinder, as shown in FIG. 1B, and that air and fuel are not mixed in an optimum ratio, and further that the fuel combustion is so undermined as to produce a large amount of exhaust fume hazardous to human heath.